By default, Mixbus is setup to "monitor" the input tracks back to the output. This means that the signal path goes into your mic, through your soundcard, through mixbus, back out your soundcard. If you want to listen to yourself through Mixbus, but you don't like to hear any latency (delay) then you should probably choose a lower "buffer size" setting in the Audio Setup window.
If you hear yourself doubled in the headphones, this means that your soundcard is setup to monitor back to you, and Mixbus is also set up to monitor. You probably only want one or the other.
If you want to hear yourself through the soundcard and NOT through Mixbus, then you should go to Preferences->Audio->Monitoring and choose "Hardware does monitoring"
You can learn more about monitoring in our manual, here: https://rsrc.harrisonconsoles.com/mixbus/mixbus32c-live-manual/8/en/topic/monitoring
Here are some more details that might be helpful:
There are several ways to set up the "input monitoring" on Mixbus, but by default, Mixbus will "monitor" the inputs of all the tracks and send them to the output when it is stopped. ( this is called Auto-Input, and can be enabled or disabled in the Transport menu ).
If you have a live microphone (such as the one on your laptop) in the same room as your speakers, then when Mixbus monitors the input, you might get a feedback squeal.
There are a lot of ways to solve this, depending on what you are trying to accomplish.
1) If you aren't going to record at all ( just mixing ) then when you start up Mixbus, in the Audio Setup dialog, just choose "None" for input.
2) use headphones to listen, instead of your speakers.
3) turn off "Auto Input" in the Transport menu
4) At the top of the mixer strip, you can force it to "Disk" which means it will not automatically switch to the inputs
5) Go to Preferences->Audio->Monitoring and choose "Hardware does monitoring"